Telephone regulators issued a series of guidelines Tuesday to help cell phone carriers meet an upcoming deadline to let customers switch to rival companies but keep their old phone numbers.

The Federal Communications Commission (FCC) said carriers should let defecting customers keep their old number even if their account has an unpaid balance. The FCC also found "no technical reason" why switching subscribers should have to wait longer than two-and-a-half hours before their old number is "ported" to their new dialing plan.

The FCC's guidelines came under immediate attack by the telephone industry. Cell phone industry lobbyists the Cellular Telecommunications & Internet Association (CTIA) claimed the FCC failed to address key questions that carriers need answered in order to meet the Nov. 24 deadline.

For example, the FCC didn't give any guidance on whether landline phone carriers are required to port their numbers to wireless carriers, the CTIA said in a statement.

"The FCC has simultaneously managed to tie the industry's hands and hold our feet to the fire--but it will be consumers who get burned after Nov. 24 if the FCC is unable to resolve these issues immediately," said Tom Wheeler, chief executive of the association.

"Now our competitors know what is expected of them: allow your customers to take their numbers when they switch wireless companies," Verizon Wireless said in a statement. The nation's largest carrier announced its support for the federal mandate several weeks ago.